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SERUM OR PLASMA TOTAL PROTEIN
Human plasma is 97% V/V water. Of the
remaining 3% occupied by solids,
over 95% is occupied by proteins. There are
more than 50 groups
of protein known to exist in plasma of which over
95% can be accounted
for by 13 proteins. These proteins includes:
(albumin, a1-antitrypsin, a1-acid glycoprotein, a
– lipoprotein,
transferrin, complements, fibrinogen, IgA, IgG
and IgM).
Function of plasma or serum proteins
Individual proteins have specific functions which
include:1- Transport, e.g.: bilirubin, metals, hormones.
2- Colloid osmotic pressure, mainly albumin.
3- Active enzymes, e.g.: complements, clotting factor.
4- Enzyme inhibitors, e.g.: antiproteases.
5- Humoral immunity, e.g.: immunoglobulins,
complements.
6- Endogenous source of aminoacids.
Synthesis and degradation
About (85) % of all plasma proteins are synthesized in the liver. The
bulk of the remainder (particularly immunoglobulins) are synthesized
by
plasma cells and cells of reticuloendothelial system while the site of
synthesis of most plasma proteins is known with some certainty; the
site
of degradation is far from clear. Most proteins are degraded by most
tissues.
Important sites of degradation include the: liver, gut, muscle and
kidney.
The control of protein synthesis and degradation is complex and often
include factors such as dietary status, circulating plasma level (feed
back), hormonal and neural.
Clinical significance
Normal range of serum total protein is 64-83 g/L (6.4-8.3 g/dl).
There are two general causes for alteration of serum total protein:
1. Change in the volume of plasma water.
2. Change in the concentration of one or more of the specific proteins.
Decrease in the volume of plasma water (haemoconcentration) as
occurs in dehydration due to inadequate water intake or to excessive
water loss due to vomiting, diarrhea or burn is reflected as
hyperproteinemia (with the concentration of all individual proteins are
increased).
Haemodilution (increase in plasma water volume) is reflected
as hypoproteinemia. This occurs with water intoxication and massive
intravenous infusions.
Of the individual serum proteins, albumin is present in such high
concentration that low level of this protein alone causes
hypoproteinemia
A mild hyperproteinemia may be caused by an increase in the
concentration of specific proteins as increase in acute phase proteins
and
polyclonal immunoglobulins as a result of infection, chronic
inflammation, chronic hepatitis and liver cirrhosis.
Marked hyperproteinemia may be caused by high levels of the
monoclonal immunoglobulin, produced in multiple myeloma.
Investigations of protein disorders
In general, the following tests are usually requested to
characterise the
pattern of abnormality. These include:1- Serum total protein.
2- Serum albumin.
3- Serum albumin: globulin ratio,
where globulin = total protein – albumin and
normal range for A:G ratio is 1.3-1.8
4- Serum protein electrophoresis where different fractions of
protein are separated.
5- Measurement of specific protein concentrations.
.
Biuret method for measuring total protein
Principle
Peptide bonds (– C – N – ) of proteins react with cupir ions
in alkaline
solution to form a violet colour product whose absorbance at
540 nm is
proportional to the concentration of protein.
Procedure
1- Set up a set of the following tubes
Test: 0.02 ml serum + 1.0 ml of Biuret reagent.
Standard: 0.02 ml of standard solution+1.0 ml of Biuret reagent.
Blank: 0.02 ml distilled water + 1.0 ml of Biuret reagent.
2- Mix, incubate for 30 min. at 20 to 25 c before reading.
3- Read absorbances of solutions at 540 nm against water.